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Combined with digital image correlation (DIC) technique, in-situ fatigue tests with different stress ratios (R = 0.1 and R = -1) at room temperature were carried out to analyze the mesoscopic surface crack propagation mechanism of selected laser melted Ni-based superalloy (SLM-IN718) in the very high cycle fatigue regime. The results show that: firstly, the DIC analysis results indicated that a plastic strain zone, similar to a butterfly shape, appeared at the crack tip when SLM-IN718 was loaded, which is consistent with the adoption of the Von Mises yield criterion; Secondly, the strain field characteristics and displacement field characteristics in front of the crack tip were analyzed to determine the existence of crack closure effect in SLM-IN718. The crack closure effect was also evaluated, i.e., for SLM-IN718, crack opened when the load reached 53% and 29% of the maximum load under R = 0.1 and R = 0, respectively; In addition, a considered the crack closure effect model was developed to evaluate the size of the plastic zone in front of the crack tip. The calculated values were in good agreement with the measured values; Finally, the surface crack propagation mechanism of SLM-IN718 under low stress conditions was suggested based on DIC analysis results.